Prior to the present invention, various types of prosthetic devices have been used for the treatment of various orthopaedic problems. Joint replacement prostheses have generally been manufactured from solid metal, for reasons of strength, and recent designs perform very well with very few cases of mechanical failure. One of the problems with solid metal implants is that their high rigidity in relation to bone can result in preferential stress transfer through the implant and away from the bone in which the implant functions. The consequence of decreased stress and strain in bone adjacent to an implant is resorptive remodeling of bone in accordance with the principles described by Wolff and Koch (Wolff, J: Das Gesetz des Transformation der Knochen. Berlin: Hirschwald, 1892; and Koch, J. L.: The Laws of Bone Architecture, Amer. J. Anat. 21:177, 1917). Loss of bone stock adjacent to a successful implant is of clinical concern and can represent a serious problem if the bone loss impairs the support and function of the implant or limits the scope of subsequent operative procedures should they ever become necessary. This problem of loss of bone stock adjacent to an implant worsens with increasing implant rigidity. Thus, the tendency for and extent of bone loss tends to increase with the use of stiffer implant materials and larger implant sizes.
To address this problem, orthopaedic implants such as joint replacement prostheses for the hip have been manufactured from two or more materials that combine to reduce the implant rigidity. One example is the "isoelastic" femoral stem designed in the early 1970's by Mathys (Mathys, R.: Stand der Verwendung von Kunststoffen fur Kunstliche Gelenke, Aktuel Traumatol., 3:253, 1973; and Mathys, R., and Mathys, R., Jr.: The Use Of Polymers For Endoprosthetic Components, in The Cementless Fixation of Hip Endoprostheses, (Morscher, E., ed.), Springer-Verlag, Berlin, 1984). It consists of an inner metal rod embedded in polyacetyl resin. The material thicknesses and properties are adjusted to render the stem similar to the femur in terms of bending stiffness. Another example is a stem with a metal core and a porous polysulfone outer surface (Demane, M., Roberson, J. R., Greenwood, K. M., Riggins, R. S., and Spector, M.: Porous Polysulfone-Coated Femoral Stems, in ASTM symposium on quantitative characterization and performance of porous implants for hard tissue applications, 1986, In press; Spector, M., Davis, R. J., Lunceford, E. M., Harmon, S. L.: Porous Polysulfone Coatings For Fixation of Femoral Stems by Bone Ingrowth, Clin. Orthop. 176:34, 1983; Spector, M., et al: Prosthetic Devices Having Coatings Of Selected Porous Bioengineering Thermoplastics. U.S. Pat. No. 4,164,794. Aug. 1979). The low stiffness of the porous polysulfone allows the manufacture of a stem with lower bending stiffness than a solid metal stem of the same dimensions. Another example is a femoral implant comprised entirely of carbon fiber reinforced composite material (Huttner, W., and Huttinger K. J.: The Use of Carbon as an Implant Material, in The Cementless Fixation of Hip Endoprostheses, (Morscher, E., ed.), Springer-Verlag, Berlin, 1984, pages 81-94; Rettig, H., and Weber, U.: Experimental and Clinical Experience With Carbon Hip Endoprostheses, in the Cementless Fixation of Hip Endoprostheses, (Morscher, E., ed.), Springer-Verlag, Berlin, 1984; and Harms, J., Mittelmeier, H., and Mausle, E.: Results of Animal Studies on the Use of Carbon Fiber-Reinforced Plastic Prostheses, in The Cementless Fixation of Hip Endoprostheses, (Morscher, E., ed.), Springer-Verlag, Berlin, 1984). With this type of design the material properties may be adjusted to render the stem more biomechanically compatible with the femur.
All of these designs possess inherent disadvantages. A common disadvantage is that the material which interfaces with bone (polymer or carbon composite) is susceptible to fretting wear against bone and the production of particulate material. The precise consequences of this from a biological perspective are not fully known. It is known, however, based on the results of studies of particulate foreign material placed in soft tissue sites in lower animals, that such material can elicit negative biological response in the form of foreign body giant cell inflammatory reactions or even neoplastic change. It is also known that polymeric wear debris generated from the articulating surfaces of joint replacement prostheses may cause soft tissue inflammatory granulomatous reactions and even bone resorption reaction which can lead to implant loosening (Buchhorn, G. H., and Willert, H. G.: Effects of Plastic Wear Particles on Tissue, in Biocompatibility of Orthopaedic Implants. Volume I, (Williams, D. F., ed.), CRC Press, Florida, 1982). In general, compared with surgical implant metals such as cobalt- and titanium-based alloys, polymeric and carbon materials are more prone to breakdown due to relative motion against bone during implant loading, may demonstrate poorer compatibility when interfaced directly with bone, and are not as well tested or understood when interfaced directly with osseous tissue because they have a much shorter history of clincial use.
A specific disadvantage with the "isoelastic" design is that the relatively small inner metal core is prone to mechanical failure. A specific disadvantage with the porous polysulfone-coated design is that the bond strength between the porous coating and the substrate is weaker than that which can be achieved by bonding porous metal to a metal substrate (Demane, M., Roberson, J. R., Greenwood, K. M., Riggins, R. S., and Spector, M.: Porous Polysulfone-Coated Femoral Stems, in ASTM symposium on quantitative characterization and performance of porous implants for hard tissue applications, 1986, in press; Pilliar, R. M., Cameron, H. U., Macnab, I.: Porous-Surfaced Layered Prosthetic Devices, J. Biomed. Eng., 10:126, 1975; and Pilliar, R. M.: Surgical Prosthetic Device With Porous Metal Coating. U.S. Pat. No. 3,855,638. Dec., 1974). A specific disadvantage with the carbon composite design is the inability to incorporate a porous surface of the type that has proven to be so efficacious for bone ingrowth fixation and long term implant stability.
Prior to the present invention, various methods have been disclosed in the literature for the attachment of prosthetic devices to the musculoskeletal system. These methods can generally be classified into those involving impaction, nails and screws, bone cement, and porous surface materials. Current interest is increasingly being focused on porous-surfaced implants designed for fixation by tissue ingrowth as representing a viable solution to the problem of late implant loosening, the most prevalent problem in joint replacement surgery using simple impaction or cementing fixation techniques. There are several types of porous materials and methods for their fabrication that have been disclosed in the literature (Pilliar, R. M.: Surgical Prosthetic Device With Porous Metal Coating. U.S. Pat. No. 3,855,638. Dec., 1974; Pilliar, R. M.: Surgical Prosthetic Device Or Implant Having Pure Metal Porous Coating. U.S. Pat. No. 4,206,516. June, 1980; Smith, L. W. et al: Prosthetic Parts and Methods of Making the Same. U.S. Pat. No. 3,314,420. Apr., 1967; Wheeler, K. R., Supp, K. R., Karagianes, M. T.: Void Metal Composite Material and Method. U.S. Pat. No. 3,852,045. Dec. 3, 1974; Frey, O.: Anchoring Surface For a Bone Implant. U.S. Pat. No. 4,272,855. June, 1981; Spector, M., et al: Prosthetic Devices Having Coatings of Selected Porous Bioengineering Thermoplastics. U.S. Pat. No. 4,164,794. Aug., 1979; Homsy, C.: U.S. Pat. No. 3,971,670. July, 1976; Tronzo, R.: U.S. Pat. No. 3,808,606. May, 1974; Sauer, B.: U.S. Pat. No. 3,986,212. Oct., 1976; and Hahn, H.: Bone Implant. U.S. Pat. No. 3,605,123. Sept., 1974). These can generally be grouped into porous polymers and porous metals. As described earlier, the porous polymers offer the advantage of allowing fabrication of a stem with lower rigidity. Their disadvantages are their generally weaker mechanical properties, their poorer biocompatibility, and their much shorter history of clinical use.
Metal implants with metal porous surfaces can be fabricated either by casting the porous surface integrally with the implant or adding the porous surface after implant casting. The method used for producing the latter type of porous surface is generally preferred because it permits more control and optimization of pore characteristics for the process of tissue ingrowth. A porous metal surface is generally added to the substrate by techniques involving high temperature diffusion bonding (Pilliar, R. M., Cameron, H. U., Macnab, I.: Porous-Surfaced Layered Prosthetic Devices. J. Biomed. Eng., 10:126, 1975; Pilliar, R. M.: Powder Metal-Made Orthopaedic Implants With Porous Surface For Fixation By Tissue Ingrowth. Clin. Orthop. 176:42, 1983; Pilliar, R. M.: Surgical Prosthetic Device With Porous Metal Coating. U.S. Pat. No. 3,855,638. Dec., 1974; and Pilliar, R. M.: Surgical Prosthetic Device or Implant Having Pure Metal Porous Coating. U.S. Pat. No. 4,206,516. June, 1980). This allows fabrication of an implant with a high strength porous coating and a coating-substrate bond strength that is stronger than achievable with porous polymer coatings (Demane, M., Roberson, J. R., Greenwood, K. M., Riggins, R. S., and Spector, M.: Porous Polysulfone-Coated Femoral Stems, in ASTM symposium on quantitative characterization and performance of porous implants for hard tissue applications, 1986, in press; Pilliar, R. M., Cameron, H. U., Macnab, I.: Porous-Surfaced Layered Prosthetic Devices. J. Biomed. Eng., 10:126, 1975; and Pilliar, R. M.: Surgical Prosthetic Device With Porous Metal Coating. U.S. Pat. No. 3,855,638. Dec., 1974). Unfortunately, the high temperature heat treatment results in microstructural change in the implant substrate which reduces the mechanical properties of the substrate material (Pilliar, R. M.: Powder Metal-Made Orthopaedic Implants With Porous Surface for Fixation By Tissue Ingrowth. Clin. Orthop. 176:42, 1983). Thus, conventional surgical grade cobalt-based alloys as well as conventional surgical grade titanium-based alloys, the two most commonly used materials for such applications, are adversely affected by adding a porous coating.
This has not precluded the clinical use of such porous coated implants (Engh, C. A., and Bobyn, J. D.: Biological Fixation in Total Hip Arthroplasty. Slack Inc., Thorofare, New Jersey, 1985). It is believed that the loss of strength of the cobalt-based alloy is tolerable (&lt;15%) and the implants used are large enough to possess adequate static and fatigue properties. Unfortunately, although larger stems are stronger they are also more rigid and thus increase the potential for stress-mediated bone loss. The preferred alternative to cobalt-based implant alloy is titanium-based implant alloy because of its lower stiffness and higher biocompatibility relative to cobalt-based alloy. However, titanium alloy is particularly notch-sensitive, meaning that cyclic loading tends to cause crack initiation at the junction of the porous material and the implant substrate. The cracks can propagate through the implant and eventually cause fatigue fracture. To overcome this problem, implant manufacturers add the porous coating by sintering at a lower temperature to reduce the strength loss in the substrate material and/or confine the porous coating to the upper or proximal portion of the stem, away from lower regions of high tensile stress which could precipitate crack initiation. Thus, the preferred titanium alloy can be used in porous-coated form if the method of manufacture is altered or the latitude in placement of the porous coating is restricted. Both these factors, however, can lead to problems. The lower sintering termperature complicates and increases the cost of manufacture and can result in a weaker bond strength between the porous coating and the substrate. Also, from a surgical perspective it is often preferred to use an implant with porous coating on the majority, not the minority of the stem length, to ensure superior clinical results. To summarize, while titanium alloy is preferred because of its lower stiffness and higher biocompatibility than cobalt alloy, the application of a porous coating can cause problems with manufacture and implant strength. While cobalt alloy can be porous-coated over any portion of the implant, it possesses inferior biocompatibility and is about 1.7 times stiffer than titanium alloy, thus increasing the potential for bone loss by stress shielding.
Considering all of these factors, it can be summarized that:
1. The concept of using a stem with flexural rigidity closer to bone than can be achieved by using solid metal is desirable for reasons of optimizing stress transfer and minimizing stress-mediated resorptive bone change.
2. But, the materials and designs currently utilized to achieve this result suffer disadvantages.
3. The concept of using a porous metal coated implant for implant fixation by tissue ingrowth is desirable for the achievement of permanent implant stability.
4. But, the preferred method of porous metal coated implant fabrication reduces implant strength, and/or choice of the preferred implant material, and/or latitude in placement of the porous coating.